Magnetic tunnel junction structure having an oxidized buffer layer and method of fabricating the same

ABSTRACT

There are provided a magnetic tunnel junction structure and a method of fabricating the same. The magnetic tunnel junction structure includes a lower electrode, a lower magnetic layer pattern and a tunnel layer pattern, which are sequentially formed on the lower electrode. The magnetic tunnel junction structure further includes an upper magnetic layer pattern, a buffer layer pattern, and an upper electrode, which are sequentially formed on a portion of the tunnel layer pattern. The sidewall of the upper magnetic layer pattern is surrounded by an oxidized upper magnetic layer, and the sidewall of the buffer layer pattern is surrounded by an oxidized buffer layer. The depletion of the upper magnetic layer pattern and the lower magnetic layer pattern in the magnetic tunnel junction region can be prevented by the oxidized buffer layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Patent Application Ser. No. 10/915,872, filed on Aug. 10, 2004, now pending, which claims priority from Korean Patent Application No. 10-2003-0055559, filed on Aug. 11, 2003, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic tunnel junction structure and a method of fabricating the same, and more particularly, to a magnetic tunnel junction structure having an oxidized buffer layer, and a method of fabricating the same.

2. Description of the Related Art

Semiconductor memory devices for storing information are normally classified into volatile memory devices and non-volatile memory devices. Whereas the volatile memory devices lose stored information therein when a power supply is shut off, the non-volatile memory devices still keep stored information even when a power supply is shut off. Such non-volatile memory devices including flash memory devices, ferroelectric random access memory (FeRAM) devices, and magnetic random access memory (MRAM) devices are typically used in memory cards, mobile phone communication terminals, and other electronics products in order to keep stored information therein, and to reduce power consumption.

The MRAM device includes a plurality of memory cells employing magnetic tunnel junction (MTJ) structures. The MTJ structure includes a lower magnetic layer pattern, a tunnel layer pattern and an upper magnetic layer pattern, which are stacked on a lower electrode. In accordance with the magnetization direction of the lower magnetic layer pattern and the upper magnetic layer pattern, there occurs a difference in the current amount flowing through the tunnel layer pattern. The MRAM device stores information by changing the magnetization direction or using the difference of the lower magnetic layer pattern and the upper magnetic layer pattern.

Conventionally, such an MTJ structure is fabricated by sequentially forming a lower conductive layer, a lower magnetic layer, a tunnel layer, an upper magnetic layer and an upper conductive layer on a semiconductor substrate, and then, sequentially patterning them using an photolithography and etching technique to form a lower electrode, a lower magnetic layer pattern, a tunnel layer pattern, an upper magnetic layer pattern and an upper electrode.

However, while sequentially etching the upper magnetic layer and the lower magnetic layer, etch residues may occur and cause a short circuit between the upper magnetic layer and the lower magnetic layer. Said short circuit results in a device failure in an MRAM device and must be avoided.

A method for avoiding a short circuit between the upper magnetic layer and the lower magnetic layer due to etch residues is taught by Chen, et al. in U.S. Pat. No. 6,165,803 in the title of “MAGNETIC RANDOM ACCESS MEMORY AND FABRICATING METHOD THEREOF,” and taught by Signorini in U.S. Pat. No. 6,485,989 in the title of “MRAM SENSE LAYER ISOLATION.” However, the method disclosed in the U.S. Pat. No. 6,485,989 causes a reduction in etch process margin because the tunnel layer becomes thinner because the upper magnetic layer should be etched such that the etching stops on the tunnel layer.

The method disclosed in the U.S. Pat. No. 6,165,803 includes etching an upper conductive layer (or mask layer), but the etching stops on an upper magnetic layer. Then, the exposed upper magnetic layer is chanced into an insulating layer as will be described below.

FIGS. 1 to 3 are cross-sectional views illustrating a method of fabricating a magnetic tunnel junction structure in accordance with the U.S. Pat. No. 6,165,803.

Referring to FIG. 1, a lower insulating layer 13 is formed on a semiconductor substrate 11. A transistor (not shown) and a digit line (not shown) are formed on the semiconductor substrate 11. The lower insulating layer 13 insulates the digit line and the magnetic tunnel junction structure. In addition, the lower insulating layer 13 has a contact hole (not shown) and a plug (not shown) to electrically connect the magnetic tunnel junction structure and the transistor.

A lower conductive layer 15, a lower magnetic layer 17, a tunnel layer 19, an upper magnetic layer 21, and an upper conductive layer 23 are sequentially formed on the lower insulating layer 13.

Referring to FIG. 2, the upper conductive layer 23, the upper magnetic layer 21, the tunnel layer 19, the lower magnetic layer 17, and the lower conductive layer 15 are sequentially patterned to form a lower electrode 15 a, a lower magnetic layer pattern 17 a, a tunnel layer pattern 19 a, a preliminary upper magnetic layer pattern 21 a and an upper conductive layer pattern, which are sequentially stacked on the lower insulating layer 13.

Next, a new mask pattern is formed on the upper conductive layer pattern. The upper conductive layer pattern is then etched using the new mask pattern as an etch mask to form an upper electrode 23 a. As a result, the upper surface of the preliminary upper magnetic layer pattern 21 a is exposed.

Referring to FIG. 3, the exposed portion of the preliminary upper magnetic layer pattern 21 a is oxidized or nitrified to form an inactive portion 21 c. As a result, a final upper magnetic layer pattern 21 b is formed in a magnetic tunnel junction region, while its sidewall is surrounded by the inactive portion 21 c. Next, an upper insulating layer 25 is formed on the resulting structure having the final upper magnetic layer pattern 21 b, and finally a bit line 27 is formed, to be electrically connected to the upper electrode 23 a.

The method disclosed in the U.S. Pat. No. 6,165,803 decreases the likelihood of a short circuit between the final upper magnetic layer pattern 21 b and the lower magnetic layer pattern 17 a by using the technique of oxidizing or nitrifying the exposed portion of the preliminary upper magnetic layer pattern 21 a.

However, this technique does have some limitations. The process for oxidizing the exposed preliminary upper magnetic layer pattern 21 a must be performed at a low temperature to protect the final upper magnetic layer pattern 21 b in the magnetic tunnel region. Therefore, this technique results in an undesirable and restrictive temperature requirement during the oxidation process.

Further, when forming the upper electrode 23 a using a photolithography and etching technique, the photoresist pattern must be removed. In the removal process, an ashing technique using an O₂ plasma gas is used. However, ashing is performed at a relatively high temperature that may exceed the aforementioned temperature requirement and thus deteriorate the final upper magnetic layer pattern 21 b.

Furthermore, to meet the temperature requirement, the upper insulating layer 25 must be formed using a low temperature process, typically lower than 300° C. However, it is difficult to adequately form the upper insulating layer with a desired high density while using this low temperature process because at the low temperature the upper insulating layer has a tendency to be formed porous. As a result of a porous formation, the oxygen atoms from the O₂ plasma used in the ashing technique may be diffused through the upper insulating layer 25 thus reaching and deteriorating the final upper magnetic layer pattern.

Accordingly, it is difficult to ensure process margins after the oxidation process and a subsequent ashing process required by this conventional technique. Embodiments of the invention address these and other limitations in the prior art.

SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention provides an improved magnetic tunnel junction structure that results in a higher process margin resulting from the subsequent ashing process.

Another feature of the present disclosure provides an improved method of fabricating a magnetic tunnel junction structure that is less likely to result in a short between an upper magnetic layer and a lower magnetic layer.

A further feature of the present disclosure provides an improved method of fabricating a magnetic tunnel junction structure being capable of ensuring a temperature margin in an oxidation process for oxidizing an upper magnetic layer. In addition the oxidation process and the ashing process occur at approximately the same time.

In one embodiment, the present disclosure provides a magnetic tunnel junction structure having an oxidized buffer layer. The magnetic tunnel junction structure includes a lower electrode, and a lower magnetic layer pattern and a tunnel layer pattern sequentially stacked on the lower electrode, and an upper magnetic layer pattern, a buffer layer pattern, and an upper electrode, which are sequentially stacked on a portion of the tunnel layer pattern. The sidewall of the upper magnetic layer pattern is surrounded by an oxidized upper magnetic layer located on the tunnel layer pattern. In addition, the sidewall of the buffer layer pattern is surrounded by an oxidized buffer layer located on the oxidized upper magnetic layer.

The oxidized buffer layer prevents oxygen atoms from deteriorating the upper magnetic layer pattern during a subsequent process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred embodiments thereof with reference to the attached drawings in which:

FIGS. 1 to 3 are cross-sectional views illustrating a conventional method of fabricating a magnetic tunnel junction structure.

FIGS. 4 to 8 are cross-sectional views illustrating a method of fabricating a magnetic tunnel junction structure according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification.

FIGS. 4 to 8 are cross-sectional views illustrating a method of fabricating a magnetic tunnel junction structure according to an embodiment of the present invention.

Referring to FIG. 4, a lower insulating layer 53 is formed on a semiconductor substrate 51. The semiconductor substrate 51 has an access transistor (not shown). The access transistor includes a source region and a drain region, which are spaced by a channel region, and also includes a gate electrode located above the channel region. The gate electrode functions as a word line. A digit line (not shown) is located above the access transistor. The digit line may be aligned in parallel with the word line.

The lower insulating layer 53 is formed on the semiconductor substrate 51 having the digit line. Further, the lower insulating layer 53 has contact holes (not shown). The contact hole may be filled with a contact plug (not shown).

A lower conductive layer 55, a lower magnetic layer 57, a tunnel layer 59, an upper magnetic layer 61, a buffer layer 63, and an upper conductive layer 65 are sequentially formed on the lower insulating layer 53.

The lower conductive layer 55 is insulated from the digit line by the lower insulating layer 53, and is electrically connected to the drain region through the contact hole, for example, using the contact plug. The lower conductive layer 55 may be a laminate of layers including a titanium layer and a titanium nitride layer, or may be a platinum group metal layer, a conductive platinum group metal oxide layer, or a combination of layers including a platinum group metal layer and a conductive platinum group metal oxide layer. For example, the lower conductive layer 55 may include a platinum layer, a ruthenium layer, an iridium layer, a rhodium layer, an osmium layer, a palladium layer, or a combination of these layers. Also, the lower conductive layer 55 may include a platinum oxide layer, a ruthenium oxide layer, an iridium oxide layer, a rhodium oxide layer, an osmium oxide layer, a palladium oxide layer, or a combination of these layers.

The lower magnetic layer 57 includes a pinning layer and a pinned layer, which are sequentially stacked. Further, the lower magnetic layer 57 may include a seed layer for controlling the crystal orientation of the pinning layer.

The pinning layer may be formed of an anti-ferromagnetic layer such as an iridium manganese (IrMn) layer or a platinum manganese (PtMn) layer.

The pinned layer may be formed of a ferromagnetic layer such as a cobalt iron (CoFe) layer, a nickel iron (NiFe) layer or a ferro manganese (FeMn) layer. The magnetization direction of the pinned layer is determined by the pinning layer, and is fixed.

The tunnel layer 59 may be an insulating layer such as an aluminum oxide (Al₂O₃) layer. In the case of forming the tunnel layer 59 as an aluminum oxide layer, the tunnel layer 59 may be formed with a thickness ranging from approximately 15 to 30 Å.

The upper magnetic layer 61 may be formed of a ferromagnetic layer such as CoFe, NiFe, FeMn layers or a combination of these layers.

The buffer layer 63 can be formed of a conductive layer such as a tantalum (Ta) layer, a titanium (Ti) layer, or a titanium nitride (TiN) layer. The buffer layer 63 may function as a capping layer for preventing the formation of a native oxide layer on the upper magnetic layer 61. The buffer layer 63 may have a thickness of about 100 Å.

Referring to FIG. 5, a photoresist pattern 67 is formed on the upper conductive layer 65 to define a magnetic tunnel junction region. The magnetic tunnel junction region is located above the digit line.

The upper conductive layer 65 is etched using the photoresist pattern 67 as an etch mask to form an upper electrode 65 a. In this case, the upper magnetic layer 61 should be prevented from being exposed by stopping the etching on the buffer layer 63. As a result, the upper electrode 65 a is formed in the magnetic tunnel junction region, and around its ambient region. Also, a region of the buffer layer 63 is exposed.

Referring to FIG. 6, the exposed buffer layer 63 is oxidized through an oxidation process. The upper magnetic layer 61 under the exposed buffer layer 63 is also oxidized together. The oxidation process may be performed by using an O₂ plasma gas at a temperature range of approximately 100 to 250° C., and preferably at a temperature of 200° C.

Since the oxidation process can be performed at a high temperature, the photoresist pattern 67 can be removed at approximately the same time through an ashing technique during the oxidation process.

The oxidation process results in an oxidized buffer layer 63 b, and a buffer layer pattern 63 a, the buffer layer pattern's sidewall surrounded by the oxidized buffer layer 63 b. Under the oxidized buffer layer 63 b, there is formed an oxidized upper magnetic layer 61 b. Further, there is formed an upper magnetic layer pattern 61 a surrounded by the oxidized upper magnetic layer 61 b and located under the buffer layer pattern 63 a.

Referring to FIG. 7, after the buffer layer pattern 63 a and the upper magnetic layer pattern 61 a are formed, the oxidized buffer layer 63 b, the oxidized upper magnetic layer 61 b, the tunnel layer 59, the lower magnetic layer 57, and the lower conductive layer 55 are sequentially patterned. As a result, there are formed a lower electrode 55 a, and a lower magnetic layer pattern 57 a and a tunnel layer pattern 59 a, which are sequentially stacked on the lower electrode 55 a, and an oxidized upper magnetic layer pattern 61 c and an oxidized buffer layer pattern 63 c, which are sequentially stacked on the tunnel layer pattern 59 a. The oxidized buffer layer pattern 63 c surrounds the sidewall of the buffer layer pattern 63 a, and the oxidized upper magnetic layer pattern 61 c surrounds the sidewall of the upper magnetic layer pattern 61 a.

The upper magnetic layer pattern 61 a is protected by the oxidized buffer layer pattern 63 c, the oxidized upper magnetic layer pattern 61 c, and the buffer layer pattern 63 a. Therefore, in the subsequent processes including the use of an ashing technique to remove a photoresist layer, the deterioration of the upper magnetic layer pattern 61 a by oxygen atoms can be prevented.

Referring to FIG. 8, an upper insulating layer 69 is formed on the semiconductor substrate having the lower electrode 55 a formed thereon. The upper insulating layer 69 is formed through a low temperature process at a temperature of lower than 300° C. in order to prevent the deterioration of the lower magnetic layer pattern 57 a and the upper magnetic layer pattern 61 a.

The upper insulating layer 69 is then patterned to form a contact hole exposing the upper electrode 65 a. Then, there is formed a bit line 71, which is electrically connected to the upper electrode 65 a. The bit line 71 is aligned to cross over the digit line.

When a magnetic RAM cell operates in a write mode, a write current flows through the digit line and the bit line 71 to magnetize the upper magnetic layer pattern 61 a. The magnetization direction aligned inside the upper magnetic layer pattern 61 a during the write operation is determined by the direction of the write current flowing through the digit line and the bit line 71. The magnetization direction aligned inside the upper magnetic layer pattern 61 a may be in parallel with, or in antiparallel with the magnetization direction maintained inside the lower magnetic layer pattern 57 a.

When the magnetized spins inside the upper magnetic layer pattern 61 a are aligned in parallel with fixed spins inside the lower magnetic layer pattern 57 a, the tunnel layer pattern 59 a shows the minimum magnetoresistance (MR_(min)). On the contrary, when the magnetized spins inside the upper magnetic layer pattern 61 a are aligned in antiparallel with the fixed spins inside the lower magnetic layer pattern 57 a, the tunnel layer pattern 59 a shows the maximum magnetoresistance (MR_(max)).

When the magnetic RAM cell operates in a read mode, a sensing voltage is applied on the bit line 71, the source region is grounded, and a read voltage is applied on the word line so as to turn on the access transistor. In the case that the tunnel layer pattern 59 a shows a low magnetoresistance depending on the magnetization direction of the upper magnetic layer pattern 61 a, a large amount of current flows through the bit line 71. On the contrary, in the case that the tunnel layer pattern 59 a shows a high magnetoresistance depending on the magnetization direction of the upper magnetic layer pattern 61 a, a small amount of current flows through the bit line 71. As a result, the magnetization direction of the upper magnetic layer pattern 61 a can be detected from the value of the current flowing through the bit line 71 under the applied sensing voltage.

EXAMPLES

The magnetoresistances of samples having the MTJ structure fabricated according to the embodiment of the present invention, as described above, in which a buffer layer was employed, and an oxidation process was performed after the buffer layer was exposed was recorded, as was the magnetoresistance of other samples having another MTJ structure fabricated by further etching the buffer layer, and exposing an upper magnetic layer, and performing an oxidation process. The measurement results are summarized as MR ratio (MRR) in Table 1. The buffer layer was formed of Ta with thickness of 100 Å, and the oxidation process was performed using an O₂ plasma gas at a temperature of 200° C. for one minute for all samples. Further, the exposed layers, buffer layer or upper magnetic layer, were overetched as shown in Table 1. The magnetoresistances were measured by applying a sensing voltage of 0.4 V. Here, the MRR can be given by Equation 1 as follows, by using Rmin of the tunnel layer pattern when the upper magnetic layer and the lower magnetic layer are in parallel with each other in their magnetization directions, and R_(max) of the tunnel layer pattern when they are in antiparallel with each other. MRR=(R _(max) −R _(min))×100/R _(min)   [Equation 1]

With a greater difference between the R_(max) and R_(min), the MRR shows a higher value. A greater difference between the R_(max) and R_(min) is advantageous because it results in a greater difference in the current flowing through the tunnel layer pattern. Thus, it is more advantageous to sense the stored information when the MRR shows a higher value. TABLE 1 exposed layer overetch MRR upper magnetic 10% 13% layer 20% 11% 30%  5% buffer layer 40% 29% (embodiment of the 60% 28% present invention) 80% 29%

As shown from the measurement results in Table 1, the MRRs of the samples according to the embodiment of the present invention, in each of which a buffer layer was exposed and an oxidation process was performed, were higher than that of the samples, in which an upper magnetic layer was exposed and an oxidation process was performed.

The samples with exposed upper magnetic layers showed considerably low values of MRR, and the values became lower as the over-etch was increased.

Further, as for the samples having the MTJ structure fabricated according to the embodiment of the present invention, their MRs were measured after an ashing process was additionally performed in order to confirm whether the upper magnetic layer pattern was deteriorated or not through a subsequent ashing process after the oxidation process was performed. The respective measurement results of the MR before and after the performance of the ashing process are shown in Table 2 as follows. The ashing process was performed using an O₂ gas, and was performed at a temperature of 110° C. for 40 minutes for the samples. TABLE 2 buffer layer MRR before MRR after overetch additional ashing additional ashing 40% 29% 28% 60% 28% 18% 80% 29% 11%

As shown in Table 2, in the sample having the buffer layer that was over-etched by 40% after its exposure, the MRRs had little difference before and after an additional ashing process. In contrast, in the sample having the buffer layer that was overetched by 60%, the MRR was considerably reduced after an additional ashing process was performed. The above reason is that the exposed buffer layer was oxidized to form an oxidized buffer layer by the oxidation process, and prevented oxygen atoms from going through into the upper magnetic layer pattern during the subsequent ashing process. However, if the thickness of the oxidized buffer layer was reduced, the oxygen atoms could not be properly prevented from penetrating through into the upper magnetic layer pattern. Therefore, it is assumed that the upper magnetic layer pattern was deteriorated during the subsequent ashing process, and MRR was reduced.

Accordingly, the present invention, performing the oxidation process after exposing the buffer layer, can prevent the deterioration of the upper magnetic layer pattern by the subsequent ashing process even with the 40% over-etch of the buffer layer.

According to the present invention, an upper magnetic layer pattern can be protected by a buffer layer pattern, an oxidized buffer layer, and an oxidized upper magnetic layer so that the deterioration of the upper magnetic layer pattern due to a subsequent ashing process is prevented. Therefore, a process margin for the subsequent ashing process can be secured. Further, according to the present invention, by performing an oxidation process for oxidizing an upper magnetic layer, a short between an upper magnetic layer and a lower magnetic layer can be prevented, and the deterioration of the upper magnetic layer even during a high temperature oxidation process can be prevented. Therefore, in the method of fabricating an MTJ structure according to the present invention, an oxidation process and an ashing process can be performed together.

Although the exemplary embodiments of the present invention have been described, it is understood that the present invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed. 

1. A structure comprising: a lower electrode; a lower magnetic layer pattern and a tunnel layer pattern sequentially stacked on the lower electrode; and an upper magnetic layer pattern, a buffer layer pattern, and an upper electrode sequentially stacked on a portion of the tunnel layer pattern, wherein a sidewall of the upper magnetic layer pattern is surrounded by an oxidized upper magnetic layer overlying the tunnel layer pattern, and wherein a sidewall of the buffer layer pattern is surrounded by an oxidized buffer layer overlying the oxidized upper magnetic layer.
 2. The structure of claim 1, wherein the lower magnetic layer pattern comprises a pinning layer and a pinned layer formed on the pinning layer.
 3. The structure of claim 1, wherein the upper magnetic layer pattern comprises ferromagnetic material.
 4. The structure of claim 1, wherein the buffer layer pattern is formed of at least one material layer selected from the group consisting of Ta, Ti, and TiN.
 5. The structure of claim 1, wherein the buffer layer pattern has a thickness of about 100 Å.
 6. The structure of claim 1, wherein the buffer layer pattern is directly below the upper electrode.
 7. The structure of claim 1, wherein the buffer layer pattern and the oxidized buffer layer are disposed at a first level.
 8. The structure of claim 1, wherein the upper magnetic layer pattern and the oxidized upper magnetic layer are disposed at a second level.
 9. A memory structure comprising: a lower magnetic layer pattern; an upper magnetic layer pattern; a tunnel layer between the lower and upper magnetic layer patterns; a buffer layer pattern overlying the upper magnetic layer pattern, wherein a sidewall of the buffer layer pattern is surrounded by an oxidized buffer layer; and an upper electrode overlying the buffer layer pattern.
 10. The memory structure of claim 9, wherein the buffer layer pattern and the oxidized buffer layer are at a first level over the lower magnetic layer pattern.
 11. The memory structure of claim 9, wherein a sidewall of the upper magnetic layer pattern is surrounded by an oxidized upper magnetic layer.
 12. The memory structure of claim 11, wherein the upper magnetic layer pattern and the oxidized upper magnetic layer are at a second level over the lower magnetic layer pattern.
 13. The memory structure of claim 11, wherein the oxidized upper magnetic layer overlies the tunnel layer.
 14. The memory structure of claim 11, wherein the oxidized buffer layer overlies the oxidized upper magnetic layer.
 15. The memory structure of claim 9, wherein the buffer layer pattern is formed of at least one material layer selected from the group consisting of Ta, Ti, and TiN.
 16. The memory structure of claim 9, wherein the buffer layer pattern has a thickness of about 100 Å.
 17. The memory structure of claim 9, wherein buffer layer pattern and the upper magnetic layer pattern are aligned with respect to the upper electrode.
 18. The memory structure of claim 9, further comprising an insulating layer overlying the oxidized buffer layer and surrounding the upper electrode.
 19. A memory structure comprising: a lower magnetic layer pattern overlying a substrate; an upper magnetic layer pattern; a tunnel layer between the lower and upper magnetic layer patterns; a buffer layer pattern overlying the upper magnetic layer pattern; and an upper electrode overlying the buffer layer pattern, wherein a sidewall of the upper magnetic layer pattern is surrounded by an oxidized upper magnetic layer overlying the tunnel layer.
 20. The memory structure of claim 19, wherein a sidewall of the buffer layer pattern is surrounded by an oxidized buffer layer overlying the oxidized upper magnetic layer. 